The present invention relates to a thermal printhead and a method of making the same.
A common thermal printhead currently in use includes a substrate formed with an electrode pattern including a common electrode and individual electrodes. The substrate is also formed with a heating resister connected to the electrode pattern. Further, the electrode pattern and the heating resister are covered and protected by a multi-layer protective coating.
In use, an outermost layer of the protective coating makes direct contact with the printing paper, and therefore worn out after repeated contacts with the paper. For example, when the printing is continued for a distance of over 100 kilometers, not only the outermost layer but also an inner layer is gradually worn by friction, eventually exposing the heating resister and the electrodes. Such a situation leads to a problem of e.g. white or black streaks found in the print.
The protective coating can be made more durable if the outermost layer is made significantly thicker. However, this causes another problem that an increased distance from the heating resister to the paper reduces thermal response, resulting in poor printing quality.
In order to avoid such a problem, a variety of non-oxide ceramics superior in a number of coating characteristics are employed as a material for the outermost layer. Among them, silicon carbide (SiC) and silicon nitride (Si3N4) are used extensively. These ceramic materials in general have a high hardness and are believed to have a superior anti-wear characteristic. For this reason, it is believed that use of these materials should allow the outermost layer to be made as accordingly thin as 4 xcexcm for increased thermal response.
However, even if such a hard material as SiC of Si3N4 is used for the outermost layer, it is still impossible to improve all film characteristics required of the outermost layer. Recently in particular, a demonstrating test has proven that even such hard materials did not provide as improved anti-wear characteristic as expected. For example, in a wearing test conducted to SiC, a thin coating of SiC was worn by repeated rubbing with a metal ball under a predetermined condition (to be detailed later). Result was, as shown in FIG. 3, that after about 1000 times of rubbing cycle, a gradual increase was found in friction coefficient, which eventually resulted in an amount of wear as much as 1.1 m. This is due to a chemical reaction at an atom-molecular level on the coating surface, in which the surface formed as SiC changed to silicon dioxide (SiO2). As for Si3N4, as will be clear from FIG. 3, its friction coefficient was found to be fairly high even at the beginning.
As described above, it is clear from the graph in FIG. 3 that the friction coefficient is a factor that determines the anti-wear characteristic and slidability, and that it is impossible to improve the anti-wear characteristic and slidability if the friction coefficient is high in general. Because of this, even if the outermost layer of the protective coating is made of such a material as SiC and Si3N4, the anti-wear characteristic and slidability cannot be improved significantly, and there is still room for research and development in the improvement of these.
On the other hand, there is another problem related to adhesion between the outermost layer and the primer layer. Specifically, if the outermost layer is formed of a common oxide ceramic, a good adhesion with the primer layer is not obtained. If the outermost layer is formed of a hard non-oxide material such as SiC and Si3N4, there is another problem that once a scratch is formed due to an external force, the coating can easily come off along the scratch because of the high hardness.
Still another problem exists in the field of electrostatic puncture. Specifically, since the oxide ceramics, SiC and Si3N4 have very low electronic conductivity, when they are slid on the printing paper, friction can cause the outermost layer to be electrically charged, which may lead to the electrostatic puncture. In order to prevent this, an electrically conducting material can be added. This has solved the problem of electrostatic puncture, but it has posed still another problem of electric corrosion that water condensation for example on the head surface causes ionization and elusion.
It is therefore an object of the present invention to provide a thermal printhead having a good slidability with respect particularly to the printing paper and an improved anti-wear characteristic, by changing material composition of the outermost layer.
Another object of the present invention is to provide a method of manufacturing such a thermal printhead.
According to a first aspect of the present invention, there is provided a thermal printhead comprising: a substrate; an electrode pattern formed on the substrate, including a common electrode and a plurality of individual electrodes; a plurality of heating dots connected to the electrode pattern; and a protective coating including a plurality of layers covering the electrode pattern and the heating dots. With the above, the protective coating includes an outermost layer composed mainly of SiC and an admixture of carbon.
Preferably, the carbon content in the outermost layer is not lower than 60 mol percent.
According to a preferred embodiment of the present invention, the protective coating includes, in addition to the outermost layer, a thick glass layer covering the heating dots and the electrode pattern, a thin glass layer formed on the thick glass layer, and an adhesion layer formed between the thin glass layer and the outermost layer. Further, the heating dots are provided by a straight thick-film resister.
According to the thermal printhead having the above construction and the arrangement, the outermost layer contains, in addition to SiC, carbon as the admixture. As a result, slidability (anti-wear characteristic) and adhesion to the primer layer can be improved by varying an amount of carbon inclusion.
According to a second aspect of the present invention, there is provided a method of manufacturing a thermal printhead comprising: a substrate; an electrode pattern formed on the substrate, including a common electrode and a plurality of individual electrodes; a plurality of heating dots connected to the electrode pattern; and a protective coating including a plurality of layers covering the electrode pattern and the heating dots. In this method, the outermost layer of the protective coating is formed by spattering with a use of a target composed mainly of SiC and an admixture of carbon.
Preferably, the carbon content in the target is 60-80 mol percent. By adjusting the carbon content within this range, film characteristic of the resulting outermost layer can be controlled.
According to a preferred embodiment of the present invention, the spattering is provided by a reactive spattering.
According to the above method of manufacture, the outermost layer is composed mainly of SiC but also include carbon as an admixture. In the outermost layer thus formed, a carbon mol percentage with respect to all of the composing atoms is slightly higher than in the equilibrium of pure SiC, which results in a various change in film characteristics of the outermost layer. Specifically, in the outermost layer composed of a mixture of pure SiC and extra carbon, even if the rubbing by the printing paper continues for a long distance, an extremely low friction coefficient is maintained for a long time. Further, the outermost layer including an admixture of carbon has a lower film stress than the layer provided by pure SiC, providing a denser layer. This improves adhesion to the primer layer and hardness, resulting in improvement in mechanical strength. Further, the outmost layer of the above arrangement has a low electric conductivity, which is not charged by the sliding friction with the paper. On the other hand, the lower conductivity causes very little electric corrosion.
The functions and advantages described above is most significant when the carbon content in the outermost layer is from 60-80 mol percent.
The carbon content in the outermost layer can be adjusted by means of a reactive spattering, in which capture of carbon into the outermost layer is controlled at an atomic level. Through this control, material composition when forming the outermost layer can be optimized.
Other characteristics and advantages of the present invention will become clearer from the following description to be presented with reference to the accompanying drawings.